AGU Journal highlights - 23 February 2005

02/23/05

I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research
Letters (GL), Journal of Geophysical Research-Atmospheres (JD), Journal of
Geophysical Research-Earth Surface (JF), and Journal of Geophysical Research-
Planets (JE). The papers related to these Highlights are printed in the next paper
issue of the journal following their electronic publication.

You may read the scientific abstract for any of these papers by going to
http://www.agu.org/pubs/search_options.shtml and inserting into the search engine
the portion of the doi following 10.1029/ (e.g., 2004GL987654). The doi is found
at the end of each Highlight, below. To obtain the full text of the research paper,
see Part II.

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1. Resolving clouds and climate

Climate simulations and numerical weather predictions rely on researchers' ability
to model the atmosphere on a global scale. Such models need to keep track of
global circulation while also accounting for the deep convection associated with
cumulus clouds, particularly in the tropics. But large-scale atmospheric models
lack the resolution to model relatively small-scale cumulus convection in detail.
Instead atmospheric models rely on parameterizations, or broad estimates of
cumulus convection that do not adequately portray the clouds. Kuang et al. resolve
these problems by reducing the scale difference between large-scale atmospheric
circulation and smaller scale convective circulation. To do this in their model, they
shrink the Earth and speed up its rotation to reduce the scale of atmospheric
circulation; they also speed up cumulus convection. This makes the scales of the
two processes similar enough to ease calculations, but not so close that it alters the
way they interact.

Title: A new approach for 3D cloud-resolving simulations of large-scale
atmospheric circulation

Volcanic activity is usually accompanied by small high-frequency earthquakes that
result from the movement of magma. These earthquakes occur because as magma
moves into a volcanic conduit, it puts pressure on the surrounding rock that, to a
limited extent, slides out of the way. Since regional stresses influence magma
movement, there may be relationships between the orientation of volcanotectonic
faults and magma movement that could prove useful to volcanologists. Diana
Roman employs numerical models to examine these relationships and finds that the
direction of movement on these strike-slip faults should be opposite to that
predicted on the basis of regional stresses. The results of this analysis do not,
however, explain the location of volcanotectonic earthquakes during the 1992
eruption at Crater Peak, Alaska. Roman speculates that the locations of preexisting
faults may be more important in influencing the location of these earthquakes.

Sea surface temperature increases in the Indian Ocean during the 1970s likely
increased precipitation in the equatorial region and may have caused a shift in the
upper ocean's oxygen content. Such a modification could explain the strong
correlation seen between the El Nino Southern Oscillation (ENSO) cycle and
isotopic oxygen found in the region's coral since then. Timm et al. analyzed the
isotopic oxygen content in coral from an archipelago in the equatorial Indian Ocean
from 1950-1994 and found that a change occurred in the 1970s that enhanced the
relationship between the oxygen content and the ENSO cycle. The authors report
that average sea surface temperatures in the region increased to nearly 28.5 degrees
Celsius [83.3 degrees Fahrenheit] in the late 1970s, which is nearly the level at
which sea surface temperature anomalies can trigger deep convection in the
tropical atmosphere. They note that other interpretations for the increased oxygen
exist, but suggest that the most likely cause is from greater precipitation.

Title: Nonstationary ENSO-precipitation teleconnection over the equatorial Indian
Ocean documented in a coral from the Chagos Archipelago

The warming trend of the world's oceans in the later half of the twentieth century
may be modulated by periods of cooling, due to natural variability of the Earth
system. In a new analysis of an expanded data set of ocean temperature profiles
dating back to 1955, Levitus et al. confirmed the ocean warming trend, which was
particularly pronounced in the Atlantic, and identified a decrease in ocean heat
content beginning around 1980. The decrease, primarily in the Pacific Ocean, was
seen in temperature records of both the uppermost 300 meters [1,000 feet] and the
layer below that down to 700 meters [2,000 feet]. The decrease has since
rebounded, and the overall warming trend continues in the new millennium. The
authors point to the increase of greenhouse gases in the atmosphere as the cause of
this long-term warming trend over the last 50 years. They attribute the large
decrease in the early 1980s to internal variability of the Earth system on decadal
time-scales.

5. Sulfur dioxide cuts could increase fine particles in the summer air

When the number of fine particles in the air increases, respiratory problems also
increase, so scientists want to identify the sources of these particles. Combustion
processes are an obvious manmade source, but there are also natural sources, such
as sea spray. Fine particles can also form, or nucleate, directly in the atmosphere.
Researchers have assumed that this process was unimportant in and around large
urban areas, because it would be overshadowed by high levels of urban particle
pollution. But, Gaydos et al. report that nucleation events actually occur frequently
in the urban eastern United States: one out of three days in Pittsburgh. By
comparing air particle measurements taken between July 2001 and January 2002
with different formation models, the researchers show that reactions between
sulphuric acid, water, and ammonia can explain the formation of these particles.
Their modeling suggests that reducing ammonia emissions reduces particle
development at any time of the year. In contrast, reductions in sulfur dioxide may
reduce or increase particle formation in the summer depending on the size of the
reduction.

Title: Modeling of in situ ultrafine atmospheric particle formation in the eastern
United States

Europe's 2003 summer heat wave fanned the flames of massive wild fires
throughout the continent, from Sweden to Russia. After the fires, severely burned
areas faced increased erosion due to the loss of vegetation and the creation of
water-repellent soils that can cause flooding, vitiate streams and lakes with mud,
and threaten people and property. The ability to predict erosion would greatly assist
efforts to limit the long-term damage caused by fires. Knowing how hot soils get
during a fire turns out to be a good way to predict erosion, according to Moody et
al. They investigated how temperature affects critical shear stress: the force it takes
to get soil moving. The scientists heated a variety of soils and measured the critical
shear stress required to erode them. They found that soils heated to more than 275
degrees Celsius [527 degrees Fahrenheit] were most likely to erode and that the
effect of soil temperature was more important than the composition of the soil.
Burn severity maps based on low-resolution infrared photographs could be used to
get a rough idea of soil temperatures. Maps based on high-resolution infrared and
other remote-sensing methods might provide a much better picture of erosion
potential, they say.

The North Pole of Mars is covered with a thick cap of water ice, over 1000
kilometers [600 miles] across and up to three kilometers [two miles] deep, that is
incised by a spiral pattern of numerous deep troughs. The walls of these canyons
reveal many alternating layers of light and dark material, thought to be layers of
water ice and dust. On Earth, changes in climate are the cause of such alternating
layers that occur in thick layers of deep-sea sediment, and these cyclical climate
changes are caused by cyclical changes in the Earth's orbit and rotation around the
Sun. By analogy, planetary scientists believe that the light and dark layers in the
Martian pole reflects changes in Martian climate and similar orbital variability.
Thanks to the availability of remarkably clear and detailed images of these layers
and detailed altimetry measurements, Milkovich and Head studied the planet using
techniques developed by paleoclimatologists to read Earth's past ice ages and
global warming events in deep sea sediment cores. They report that one particular
pattern with a wavelength of about 30 meters [100 feet] can be traced across at
least three-fourths of the ice cap and seems to reflect changes in the orbit of Mars
that happened every 51,000 years. They also identify a unit that may relate to Mars'
most recent ice age, which occurred between 500,000 and two million years ago.